The present invention generally relates to a semiconductor memory circuit having an ohmic contact between an aluminum-silicon alloy metallization film and a silicon substrate.
Generally, an alloy of aluminum and silicon is used for forming source and drain electrodes of a transistor provided in a peripheral circuit portion (including a sense amplifier, an address decoder, a logic circuit and the like) of a semiconductor memory device. The source and drain electrodes are in contact with respective active layers formed in a silicon substrate through respective contact holes. Such a contact is an ohmic contact. Generally, aluminum-silicon alloy makes a low-resistance ohmic contact with the silicon substrate.
It is known that excessive silicon atoms contained in an aluminum-silicon alloy film may precipitate in a contact hole due to heat treatment and that precipitation of excessive silicon atoms increases ohmic contact resistance. It is also known that the use of a barrier layer such as a tungsten silicide (WSi.sub.x) film and a molybdenum silicide (MoSi.sub.x) film, provides an effective solution for preventing precipitation of silicon atoms. Usually, such a barrier layer is provided so as to underlie an aluminum-silicon alloy film, or cover an exposed surface of a silicon substrate in a contact hole.
On the other hand, the use of the barrier layer may lead to an increased number of manufacturing steps of the memory device as well as a decrease of the yield rate. Therefore, the memory device which employs the barrier layer may be expensive.
Generally, electric characteristics of a conventional semiconductor memory device are not greatly affected by precipitation of silicon. In other words, excessive silicon atoms do not precipitate greatly in a contact hole having a normal size obtainable by conventional photolithography technology. Therefore, some marketed semiconductor memory devices do not have the barrier layer. However, as described later, precipitation of silicon atoms increases as a contact area where the aluminum-silicon alloy film is in contact with the substrate decreases. Therefore, an economical solution which can provide a low-resistance ohmic contact without additional steps nor a decrease of the yield rate is desired.